Compounds with a nitrogen atom attached to a carbonyl group, thus having the structure R–CO–NR′R″, are called amides and have different chemical properties from amines.

Classes of amines

An aliphatic amine has no aromatic ring attached directly to the nitrogen atom.[6]Aromatic amines have the nitrogen atom connected to an aromatic ring as in the various anilines. The aromatic ring decreases the alkalinity of the amine, depending on its substituents. The presence of an amine group strongly increases the reactivity of the aromatic ring, due to an electron-donating effect.

Secondary amines — Secondary amines have two organic substituents (alkyl, aryl or both) bound to the Nitrogen together with one hydrogen (or no hydrogen if one of the substituent bonds is double). Important representatives include dimethylamine and methylethanolamine, while an example of an aromatic amine would be diphenylamine.

Tertiary amines — In tertiary amines, all three hydrogen atoms are replaced by organic substituents. Examples include trimethylamine, which has a distinctively fishy smell, or triphenylamine.

Cyclic amines — Cyclic amines are either secondary or tertiary amines. Examples of cyclic amines include the 3-membered ring aziridine and the six-membered ring piperidine. N-methylpiperidine and N-phenylpiperidine are examples of cyclic tertiary amines.

It is also possible to have four organic substituents on the nitrogen. These species are not amines but are quaternary ammonium cations and have a charged nitrogen center. Quaternary ammonium salts exist with many kinds of anions.

Naming conventions

Amines are named in several ways. Typically, the compound is given the prefix "amino-" or the suffix: "-amine". The prefix "N-" shows substitution on the nitrogen atom. An organic compound with multiple amino groups is called a diamine, triamine, tetraamine and so forth.

Systematic names for some common amines:

Lower amines are named with the suffix -amine.

methylamine

Higher amines have the prefix amino as a functional group. IUPAC however does not recommend this convention, but prefers the alkanamine form, e.g. pentan-2-amine.

2-aminopentane(or sometimes: pent-2-yl-amine or pentan-2-amine)

Physical properties

Hydrogen bonding significantly influences the properties of primary and secondary amines.[7] Thus the melting point and boiling point of amines is higher than those of the corresponding phosphines, but generally lower than those of the corresponding alcohols and carboxylic acids . For example, methyl and ethyl amines are gases under standard conditions, whereas the corresponding methyl and ethyl alcohols are liquids. Gaseous amines possess a characteristic ammonia smell, liquid amines have a distinctive "fishy" smell.

Also reflecting their ability to form hydrogen bonds, most aliphatic amines display some solubility in water. Solubility decreases with the increase in the number of carbon atoms. Aliphatic amines display significant solubility in organic solvents, especially polar organic solvents. Primary amines react with ketones such as acetone.

The aromatic amines, such as aniline, have their lone pair electrons conjugated into the benzene ring, thus their tendency to engage in hydrogen bonding is diminished. Their boiling points are high and their solubility in water is low.

Chirality

Inversion of an amine. The pair of dots represents the lone electron pair on the nitrogen atom.

Amines of the type NHRR′ and NRR′R″ are chiral: the nitrogen atom bears four substituents counting the lone pair. The energy barrier for the inversion of the stereocenter is relatively low, e.g., ~7kcal/mol for a trialkylamine. The interconversion of the stereoisomers has been compared to the inversion of an open umbrella into a strong wind. Because of this low barrier, amines such as NHRR′ cannot be resolved optically and NRR′R″ can only be resolved when the R, R′, and R″ groups are constrained in cyclic structures such as aziridines. Quaternary ammonium salts with four distinct groups on the nitrogen are capable of exhibiting optical activity.

The nitrogen atom features a lone electron pair that can bind H+ to form an ammonium ion R3NH+. The lone electron pair is represented in this article by a two dots above or next to the N. The water solubility of simple amines is largely due to hydrogen bonding between protons in the water molecules and these lone electron pairs.

The +I effect of alkyl groups raises the energy of the lone pair of electrons, thus elevating the basicity. Thus the basicity of an amine may be expected to increase with the number of alkyl groups on the amine. However, there is no strict trend in this regard, as basicity is also governed by other factors mentioned above. Consider the Kb values of the methyl amines given above. The increase in Kb from methylamine to dimethylamine may be attributed to the +I effect; however, there is a decrease from dimethylamine to trimethyl amine due to the predominance of steric hindrance offered by the three methyl groups to the approaching Brønsted acid.

The −M effect of aromatic ring delocalises the lone pair of electrons on nitrogen into the ring, resulting in decreased basicity. Substituents on the aromatic ring, and their positions relative to the amine group may also considerably alter basicity as seen above.

The solvation of protonated amines changes upon their conversion to ammonium compounds. Typically salts of ammonium compounds exhibit the following order of solubility in water: primary ammonium (RNH+3) > secondary ammonium (R2NH+2) > tertiary ammonium (R3NH+). Quaternary ammonium salts usually exhibit the lowest solubility of the series.

In sterically hindered amines, as in the case of trimethylamine, the protonated form is not well-solvated. For this reason the parent amine is less basic than expected. In the case of aprotic polar solvents (like DMSO and DMF), wherein the extent of solvation is not as high as in protic polar solvents (like water and methanol), the basicity of amines is almost solely governed by the electronic factors within the molecule.

Synthesis

Alkylation

The most industrially significant amines are prepared from ammonia by alkylation with alcohols:

ROH + NH3 → RNH2 + H2O

These reactions require catalysts, specialized apparatus, and additional purification measures since the selectivity can be problematic. The same amines can be prepared by treatment of haloalkanes with ammonia and amines:

RX + 2 R′NH2 → RR′NH + [RR′NH2]X

Such reactions, which are most useful for alkyl iodides and bromides, are rarely employed because the degree of alkylation is difficult to control.[5] Selectivity can be improved via the Delépine reaction, although this is rarely employed on an industrial scale.

Reductive routes

Via the process of hydrogenation, nitriles are reduced to amines using hydrogen in the presence of a nickel catalyst. Reactions are sensitive to acidic or alkaline conditions, which can cause hydrolysis of the –CN group. LiAlH4 is more commonly employed for the reduction of nitriles on the laboratory scale. Similarly, LiAlH4 reduces amides to amines. Many amines are produced from aldehydes and ketones via reductive amination, which can either proceed catalytically or stoichiometrically.

Aniline (C6H5NH2) and its derivatives are prepared by reduction of the nitroaromatics. In industry, hydrogen is the preferred reductant, whereas in the laboratory, tin and iron are often employed.

Specialized methods

Many laboratory methods exist for the preparation of amines, many of these methods being rather specialized.

Similarly, with sulfonyl chlorides, one obtains sulfonamides. This transformation, known as the Hinsberg reaction, is a chemical test for the presence of amines.

Because amines are basic, they neutralize acids to form the corresponding ammonium salts R3NH+. When formed from carboxylic acids and primary and secondary amines, these salts thermally dehydrate to form the corresponding amides.

Diazotization

Amines react with nitrous acid to give diazonium salts. The alkyl diazonium salts are of little synthetic importance because they are too unstable. The most important members are derivatives of aromatic amines such as aniline ("phenylamine") (A = aryl or naphthyl):

Anilines and naphthylamines form more stable diazonium salts, which can be isolated in the crystalline form.[10] Diazonium salts undergo a variety of useful transformations involving replacement of the N2 group with anions. For example, cuprous cyanide gives the corresponding nitriles:

Aryldiazonium couple with electron-rich aromatic compounds such as a phenol to form azo compounds. Such reactions are widely applied to the production of dyes.[11]

Application of amines

Dyes

Primary aromatic amines are used as a starting material for the manufacture of azo dyes. It reacts with nitrous acid to form diazonium salt, which can undergo coupling reaction to form azo compound. As azo-compounds are highly coloured, they are widely used in dyeing industries, such as: